[PPC] Convert tests to check 'target=<triple>'

Two tests checked 'ppc64be' which appears not to exist; the tests
pass on clang-ppc64be-linux-multistage so I assume they are fine
and just removed those UNSUPPORTED lines. All others were converted
to the new target= format, and get the same results on ppc bots as
before.

Part of the project to eliminate special handling for triples in lit
expressions.

Differential Revision: https://reviews.llvm.org/D138954